A typical finished vehicle body will have three layers of paint coatings: clear, color and prime. For best quality and process control, it is desirable to measure each layer. Traditional measuring instruments used in vehicle manufacturing facilities are based on a magnetic principle and determine the total thickness of paint on a steel substrate. This is not applicable to plastic substrates which are becoming increasingly important in automotive fabrication. Moreover, only the total thickness is measured rather than the individual layers. To make layer measurements on plastic automotive panels it has been necessary to fasten steel coupons to a plastic buck representing a body, painting the buck and removing the coupons after each layer of paint to measure the paint thickness increment by the magnetic method. This method is very slow and expensive, and, as a practical matter, can not be used very often.
It is known that ultrasonic measurement can reveal thicknesses of materials by transmitting a high frequency sound pulse through the material, receiving the echo pulses reflected from the surface and subsequent interface layers of the material, measuring the time between pulses, and multiplying half the time by the velocity of sound in the material. This principle has been applied to measuring multiple layers of plastic having thickness in the range of 0.001 to 0.005 inch as described in the Panametrics brochure, "NDT Applications" No. 3, February 1986. There, a 50 or 100 MHz transducer is used with a short delay line coupling the transducer to the material surface. Received waveforms are displayed on an oscilloscope and are visually inspected to identify the echoes and measure the time separation of pulses. In addition, this method has been applied to the measurement of mulitples layers of paint in a vehicle manufacturing environment, with an average of several repeated waveforms displayed on an oscilloscope and visually compared by a trained operator to predetermined sample data for identification of pulse echoes and measurement, on the oscilloscope screen, of pulse echo delays. This practice requires a good instinct for interpreting waveforms and repeated accurate measurement by a human operator. To reduce the chance of error and obtain more consistently accurate measurements, a method of automatically interpreting the waveform and making the measurement is desirable.